holocene paleoenvironmental changes from the romanian black sea ...

Geology

LATE PLEISTOCENE - HOLOCENE PALEOENVIRONMENTAL CHANGES FROM THE ROMANIAN BLACK SEA SHELF INFERRED BY MICROFAUNAL AND ISOTOPE FLUCTUATIONS Dr. Andrei Briceag1 PhD Student Anastasia Yanchilina2 Prof. Dr. William B.F. Ryan2 Dr. Gheorghe Oaie1 Dr. Mihaela C. Melinte-Dobrinescu1 1 2

National Institute of Marine Geology and Geo-ecology GeoEcoMar, Romania Lamont-Doherty Earth Observatory of Columbia University, USA

ABSTRACT The transition of the Black Sea from an inland lake to a marine basin during the last glacial/deglacial episode is still generating discussion in the scientific community. Qualitative and quantitative study of the ostracod and foraminiferal assemblages coupled with isotopic (oxygen) and calcium carbonate (CaCO 3), were performed on an AMS 14C dated core, 09 SG 13, collected from the Romanian Black Sea outer shelf (200 m water depth). The present study documents the changes that occur in the Black Sea from the Last Glacial Maximum towards the Present. In the 09 SG 13 core, two lithological units, respectively the youngest Unit 1 (The Coccolith mud) and the oldest Unit 3 (The Lacustrine lutite), were identified. The Unit 2 (The Sapropel Mud) is missing. In the two lithological units identified, four ecobiozones are distinguished based on changes in microfossil assemblages, which reveal climate shifts and oscillations of salinity. The Last Glacial Maximum period is characterized by stable conditions recorded by homogenous ostracod assemblages. However, the onset of the Fennoscandian Meltwater Pulses changed drastically the hydrological conditions of the basin, by lowering the salinity and allowing the fresh water ostracods to thrive. These fresh water pulses are mirrored in the sedimentary record by the reddish-brown clay deposition. This study documents also a core from the shallow area of the Black Sea inner shelf, MN 103_04 (78 m water depth). Based on macro and microfauna assemblages, this core reveals sea level changes that took place during the last approximately 6,000 yr BP. Keywords: ostracods, foraminifers, sea level fluctuation, reddish-brown clay. INTRODUCTION The Black Sea is a semi-enclosed marginal basin which connects with the Mediterranean Sea through the Bosphorus Strait, Marmara Sea and Dardanelles Strait. Within the Quaternary, the Black Sea experienced a series of sea level fluctuations driven by the global glaciations and deglaciations, which led to successive episodes of isolation followed by a re-connection with the Mediterranean Sea [1], [2]. In the Late

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16th International Multidisciplinary Scientific GeoConference SGEM 2016

Pleistocene Holocene interval, the water level of the Black Sea was controlled by the regional climatic modifications rather than the global eustatic changes [3]. The Black Sea eustatic level during the Last Glacial Maximum (LGM, between ~ 25,000 18,000 yr BP) was at least 100m lower than in the Present and the isolated basin had a brackish-freshwater character [4], [5]. In recent years great advances have been made in the understanding of the reconnection between Mediterranean and the Black Sea. There are still many debates in the scientific community regarding the mechanism and the timing of reconnection. Three main hypotheses have been advanced so far for the way of the reconnection between the two marine basins: (a) The Flood Hypotheses [5] [5] argues for a catastrophic reco cm/day sea level rise. The authors postulated that this catastrophic event took place when the Mediterranean water level has increased and broke the hypothetical sediment dam across the Bosphorus Strait at ~ 7,150 14C yr BP [5]. ]. They reported mud cracks at ~ -100 m and vegetation roots at -123 123 m and presumed that the shelf was exposed prior to 8,500 14C yr BP; (b) The Outflow Hypotheses [6], [6], [7] [7] postulated that there was no [7 catastrophic flood duringg the reconnection between the two marine basins; hence the reconnection was gradual and progressive. Therefore at ~ 11,900 14C yr BP there was an outflow of the Black Sea waters through Bosphorus Strait in the Mediterranean [6], [ [7], 14 while between ~ 8,500 and 8,000 C yr BP the two way water circulation between the Black Sea and the Mediterranean was established estab lished through Bosphorus Strait; (c) The Oscillating Sea-level level Hypotheses [8] argues that the reconnection of the Black Sea to the Mediterranean Sea tookk place in an oscillating manner that allowed successive waves of Mediterranean immigrants to enter the Black Sea basin during six transgression-regression regression stages. The aforementioned author believes that the first connection between the Black Sea and Mediterranean Medit erranean may not have been through the Bosphorus Strait, but through alternative routes, such as Izmit Bay Sapanca Lake Sakarya River. In the Black Sea Holocene sediments [9], three distinct lithological units were identified in the deep parts of the basin: the oldest unit, Unit 3, the lacustrine lutite, deposited during the freshwater or oligohaline stage; Unit 2, the sapropel mud, corresponding to a brackish, anoxic phase, and the youngest unit, Unit 1, the microlaminated coccolith ooze, a pelagic sediment deposited under recent marine conditions, associated with the full invasion of the coccolithophoridae species Emiliania huxleyi. In the shallow setting of the NW Black Sea [10] identified a Shallow Unit, corresponding to Unit 1 and Unit 2 deposited in the deeper parts of the basin. The integration of pre-existing and new datasets has allowed an unprecedented degree of detail in the succession of events which preceded and succeeded the transition from a fresh-brackish water character to a marine one. At this stage, further refinement is achievable only if there is a consistent and rigorous application of analytical best practices as well as a thorough and unbiased integration of all available investigation tools. In this study detailed micropaleontological analyses coupled with oxygen isotopes, CaCO3 values and AMS 14C dating are used for palaeoenvironmental reconstructions of the last 25,000 years.

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Geology

MATERIAL AND METHODS For this study, two undisturbed cores collected with RV Mare Nigrum were analyzed. , was retrieved from One gravity core, the N Romanian Black Sea outer shelf, from 200m water depth. The second core, MN was collected using a multicorer from 78 m water depth (Fig. 1). In order to obtain a good resolution, in the 09 SG 13 core 100 micropaleontological samples were acquired at each 2 cm interval. The MN 103_04 core was continuously sampled, each sample covering 3 cm of the core. Quantitative and qualitative studies were achieved for ostracod, foraminiferal and calcareous nannoplankton assemblages for both cores (Fig. 2). For the 09 SG 13 core the micropaleontological investigations were integrated with sedimentological analyses, 18 O isotopes, CaCO3 measurements and AMS 14C dating (Fig. 2). The micropaleontological, sedimentological and CaCO 3 analyses were achieved at 18 O isotopes and AMS 14C dating were undertaken at Lamont Doherty Earth Observatory of Columbia University and NOSAMS Facility at Woods Hole Oceanographic Institution.

Fig. 1. Location of the analyzed cores. RESULTS The lithology of the 09 SG 13 core consists of two main lithological units, as described by [9], as follows: Unit 3 from the base of the core, i.e. 396 cm bsf (below sea floor), to 7 cm bsf, and Unit 1, from 7 cm bsf to the top of the core. Between these two lithological units, an erosional surface can be assumed as the Unit 2, i.e. the Sapropel Mud, was not identified. Between 396 cm bsf and 160 cm bsf, the lithology consists of grey and green soft clays with specks of iron sulfide (FeS), fine siltic laminae and mm up to cm thick sand intercalations. The 160 to 53 cm bsf interval is characterized by the

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deposition of reddish-brown clays with cm thick sand intercalations (Fig. 2). The youngest deposits of Unit 3, between 53 and 7 cm bsf, are composed of grey and green soft clays with black specks of organic matter. The macrofauna identified in the Unit 3 is represented by bivalve broken shells of Dreissena rostriformis and by gastropod shells. The uppermost part of the core, i.e. between 7 and 0 cm bsf, is characterized by the deposition of grey and green silty clays, interbedded with millimeter-scale coccolith white laminae. For the age model of the 09 SG 13 core, four lithological boundaries were dated using 14 C ages of juvenile Dreissena rostriformis:: 389 cm bsf for the base of the gray clay, 159 cm bsf for the boundary between the gray clay and the red clay, 59 cm bsf for the boundary between the red clay and the light gray clay, and 13 cm bsf for the transition from light gray clay to white clay (Fig. 2).

Fig. 2. Lithology and microfaunal (ostracods and foraminifers) distribution and 18 O isotopes and CaCO3 fluctuations during abundance in the 09 SG 13 core. 14 the last 25,000 yr BP. Legend: Lithology: 1 Greenish grey silty clays with fine coccolith laminae intercalations; 2 Greenish grey silty clays; 3 Grey silty clays; 4 Fine silty sands; 5 Reddish-brown clay; 6 - FeS and iron oxides; 7 - Organic matter. Relative abundance of species: a Abundant; b Frequent; c Moderate; d Rare. The microfauna identified in Unit 3 of 09 SG 13 core contains a total number of 14 Caspian in origin ostracod species, belonging to 8 genera. The 396 to 160 cm bsf interval is dominated by the Candonidae ostracod species Candona schweyeri and C. fabaeformis. The other ostracod species recorded in this interval with lower abundances are: Euxinocythere bosqueti, Loxoconcha lepida, Amnicythere olivia, Erpetocypris sp., 308

Geology

Amnicythere striatocostata, Leptocythere gracilloides, Amnicythere cymbula, Ilyocypris sp., Cyprideis torosa and Amnicythere caspia. From 160 to 53 cm bsf (the reddishbrown clay interval) the diversity of the ostracod assemblages is increasing and the Loxoconchidae and Leptocytheridae are dominating. The maximum diversity (10 species) is recorded in the upper part of Unit 3 (between 53 and 7 cm bsf) where the Leptocytheridae are dominating. In the Unit 3 no foraminifera taxa in situ was identified. In the uppermost part of the core, in the Unit 1, there is a shift from the Caspian in origin ostracods to the Mediterranean ones. The ostracod species that are dominating this interval are Hiltermannicythere rubra, Cytheroma variabilis and Palmoconcha granulata (Fig. 2). This lithological unit is marked also by the appearance of well-preserved foraminifers represented by Ammonia tepida, tepida, Cribroelphidium poeyanum and Textularia cushmani taxa (Fig. 2) and by blooms of the calcareous nannoplankton species Emiliania huxleyi. Based on the inorganic CaCO3 fluctuations, three intervals were separated in the 09 SG 13 core. From 396 to 200 cm bsf the values are comprised between 5.0 and 7.3%, from 200 to 53 cm bsf the values are decreasing to 2.1 to 3.5%, and from the top of the reddish-brown clays (i.e. 53 cm bsf) to the top core, the CaCO 3 values are increasing to 14.0% (at 32 cm bsf), reaching values as high as 15.2% (at 7 cm bsf) in the lowermost 18 part of Unit 1 (Fig. 2). O isotopes were measured on juvenile Dreissena rostriformis shells for the surface signal and on ostracod valves for the bottom signal. From 396 cm bsf to the onset of the reddish-brown clays (i.e. 160 cm bsf) the values derived from D. rostriformis shells are constant, around values from the reddish-brown clays (between 160 and 53 cm bsf) are decreasing to and then 18 18

O values derived from ostracod valves are comprised between -

the core,

The lithology of the MN 103_04 core consists of grey muds with a 2 cm black fluffy mud at the top part (Fig. 3). The grey muds are very rich in gastropods and bivalves. The most abundant gastropod species are Cerithidium pusillum and Rissoa spp. The bivalves are represented by Modiolus phaseolinus, phaseolinus Mytilus galloprovincialis, Abra alba and Cardium spp [11]. [1 The ostracod fauna from MN 103_04 core consists of 7 species belonging to 7 genera. The ostracod species are re represented by Leptocythere multipunctata, granulata, Cytheroma variabilis, Callistocythere diffusa, multipunctata Palmoconcha granulata Xestoleberis cornelii, cornelii Cythereis rubra pontica and Paradoxostoma simile [11]. The macro and microfauna of this core is discussed in detail in [11]. [1 For the age model of MN 103_04 core, the changes of macrofauna assemblages were correlated with the ones from B2KS24 core of [12]. [1 The aforementioned authors used 14C absolute ages for the transition from Mytilus to Modiolus facies (Fig. 3). DISCUSSION For a better understanding of the Late Pleistocene - Holocene paleoenvironmental changes from the Romanian Black Sea shelf two cores were studied. The 09 SG 13 core documents especially the lacustrine stage of the Black Sea, while the MN 103_04 point

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out the paleoenvironmental changes that took place after the reconnection of the Black Sea with the Mediterranean Sea.

Fig. 3. A Lithology, bathymetry interpretation and macrofaunal distribution and [12]. B - Ostracods abundance in the MN 103_04 core. The B2KS24 core after [1 distribution and abundance in the MN 103_04 core. Based on the ostracod and foraminifera assemblages, assemblages 4 Ecobiozones were identified in the 09 SG 13 core. Ecobiozone 4, identified between 396 and 160 cm bsf, is characterized by the dominance of the Candonidae species that prefer salinities The presence of the cold water species Candona fabaeformis and Ilyocypris sp. and the low ostracod diversity argues for a period with low bottom water temperature. This is supported by the relatively low and homogenous 18 O and CaCO3 values. This interval corresponds to the Last Glacial Maximum (LGM) and Heinrich Stadial 1 (HS1), a period with colder and dryer conditions coupled with low continental hydrologic input [13]. Ecobiozone 3 corresponds to the reddish-brown clay depositional interval and is characterized by an increase in ostracod abundance and diversity and a shift from the Candonidae dominance to Loxoconchidae and Leptocytheridae. The increase in ostracod abundance and diversity suggest a high nutrient input into the basin. This interval corresponds to the beginning of the 310

Geology

Fennoscandian Ice Sheet meltwater pulse (at 15,000 yr BP) up to the end of the meltwater pulse (at 13,800 yr BP). The occurrence with high abundances of the oligohaline ostracod species Loxoconcha lepida and Amnicythere cymbula throughout this interval suggests a decrease in salinity from 54]. Ecobiozone 2 is characterized by the highest abundance and diversity of ostracods in the core and by the presence of the warm water species Erpetocypris sp.. This interval corresponds to the 18 O isotope and CaCO3. Ecobiozone 1 is marked by the appearance of the foraminifera species Ammonia tepida and by the occurrence of the Mediterranean in origin ostracod species that prefer salinities of 17with wi th blooms of the calcareous nannoplankton species Emiliania huxleyi. In the MN 103_04 core there is a shift from the Mytilus to the Modiolus mollusk facies. According to [12] this shift took place at around 5.000 yr BP (Fig. 3). We can assume that our base of the core is at approximately 6.000 yr BP. Taking into consideration that the mollusk Mytilus galloprovincialis is currently living at water depths of maximum 20 m, and Modiolus phaseolinus prefers water depths of approximately 70 m, a gradual sea level evel rise from 20 to 78 m water depth (the water depth of the core location) could be assumed during the last 6.000 yr BP. This is supported by the decrease in ostracod abundance from the base to the top of the core (Fig. 3). CONCLUSION 18

O and CaCO3 records constrained by 14C dating obtained from two cores collected from the Romanian Black Sea shelf reveal major paleoenvironmental changes that took place during the last 25,000 yr BP. Based on the presented data, we assume that the LGM period (between 25,000 and 15,000 yr BP) was characterized by stable climatic and basin salinity conditions (with values of 5). The beginning of the Fennoscandian Ice Sheet meltwater pulse at 15,000 yr BP has been decrease the Black Sea basin salinity. This is reflected in the ostracod assemblages by high diversity and abundance of the Loxoconchidae and Leptocytheridae fresh water species. This increase in abundance and diversity reached the maximum values during eptocytheridae species are dominating the ostracod assemblage. The high values of CaCO3 indicate enhanced primary productivity during this warm period. From the Late Holocene onwards, stable marine conditions set in over the Black Sea basin. The macro and microfaunal shift suggests a gradual sea level rise, from around 20 m to 78 m water depth, during the last 6,000 yr BP.

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[3] Lericolais G., Guichard F., Morigi C., Minereau A., Popescu I., Radan S., A post Younger Dryas Black Sea regression identified from sequence stratigraphy correlated to core analyses and dating, Quaternary International, 225, pp 199-209, 2010. [4] Fedorov P.V., Postglacial transgression of the Black Sea, International Geology Review 14 (2), 160 164, 1971. [5] Ryan W.B.F., Pitman W.C., Major C.O., Shimkus K., Moskalenko V., Jones G.A., Marine Geology, 138, pp 119-126, 1997. Marmara Sea and vigorous outflow into the Aegean Sea from the Marmara Sea-Black Sea drainage corridor, Marine Geology 153, pp 275-302, 1999. [7] Hiscott R.N., Aksu A.E., Mudi Rochon A., The Marmara Sea Gateway since ~16 ky BP: non-catastrophic causes of paleoceanographic events in the Black Sea at 8.4 and 7.15 ky BP, In: Yanko-Hombach, V., Gilbert, A.S., Panin, N., Dolukhanov, P.M. (Eds.), The Black Sea Flood Question: Changes in Coastline, Climate, and Human settlement, Springer, Dordrecht, pp 89-117, 2007. [8] Yanko-Hombach V., Controversy over Noah's Flood in the Black Sea: geological and foraminiferal evidence from the shelf, In V. Yanko-Hombach, A.S. Gilbert, N. Panin, & P.M. Dolukhanov (Eds.), The Black Sea flood question: Changes in coastline, climate and human settlement, Heidelberg: Springer, pp 149-203, 2007. [9] Ross D.A., Degens E.T., Recent sediments of the Black Sea. In: Degens E.T. and Ross D.A. (Eds.), The Black Sea: Geology, Chemistry, and Biology, American Association of Petroleum Geologists, Tulsa, USA, pp 183-199, 1974. [10] Giunta S., Morigi C., Negri A., Guichard F., Lericolais G., Holocene biostratigraphy and palaeoenvironmental changes in the Black Sea based on calcareous nannoplankton, Marine Micropaleontology 63, pp 91-110, 2007. [11]] Briceag A., Ion G., Holocene ostracod and foraminiferal assemblages of the Romanian Black Sea shelf, Quaternary International 345, pp 119-129, 2014. [12] Lericolais G., Bulois C., Gillet H., Guichard F., High frequency sea level [12] fluctuations recorded in the Black Sea since the LGM, Global and Planetary Change 66, pp 65-75, 2009. [13] 3] Soulet G., Menot G., Garreta V., Rostek F., Zaragosi S., Lericolais G., Bard E., Hydrologic and climatic implications, Earth and Planetary Science Letters 308, pp 245-258, 2011. [14] Ivanova E.V., Murdmaa I.O., Chepalyga A.L., Cronin T.M., Pasechnik I.V., [14] Levchenko O.V., Platonova E.A., Holocene sea-level oscillations and environmental changes on the Eastern Black Sea shelf, Palaeogeography, Palaeoclimatology, Palaeoecology, 246, pp 228-259, 2007.

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